util-linux 2.42.3 fixes mount races and nsenter leaks

util-linux 2.42.3 ships security fixes for four newly assigned CVEs in mount, nsenter, and unshare, plus a follow-on hardening for an older wall and write flaw.

CVE-2026-76642 covers post-mount hooks that still ran after an external mount helper failed. Options such as idmap and ownership or mode changes could therefore act on whatever filesystem already sat at the target path, as if the mount had succeeded.

CVE-2026-78410 is a time-of-check time-of-use race on the source path in restricted (SUID, non-root) mounts. Canonicalization and open_tree handling followed symlinks through user-writable directories while privileged, so a local attacker could redirect a privileged mount or a post-mount ownership change to an arbitrary path.

CVE-2026-78409 addresses an X-mount.subdir escape: open_tree followed symlinks in intermediate components when opening a subdirectory on a detached mount, letting the operation leave the intended tree.

CVE-2026-78408 notes that nsenter and unshare created file descriptors without O_CLOEXEC, risking leakage across exec. The release adds the flag as defense in depth; distributor packaging may still need a further stable-branch correction for that change.

Separately, wall and write received another fix related to CVE-2024-28085. An earlier sanitization covered message bodies only; banner headers still interpolated the system hostname unsanitized, so an unprivileged user could inject terminal escape sequences through a user-namespace hostname. Skyler Ferrante reported that gap.

The mount issues matter most on systems that expose setuid mount helpers or allow unprivileged users to influence mount paths and post-mount options. Operators should move to 2.42.3 or equivalent backports once available.